In state of the art semiconductor device processing, lithography is a key technology in fabricating wafers with large numbers of chip sites. Although x-ray and electron beam lithography tools are useful and effective, especially for mask making, photolithography remains the most widely used lithography tool for semiconductor wafer fabrication. Likewise, although a variety of exposure tools have been developed over the years, step and repeat cameras are essentially ubiquitous in commercial integrated circuit manufacture. These tools, usually referred to as steppers, rely for their effectiveness on the ability to register photomasks in exact alignment from level to level. Typical state of the art wafer fabrication processes have several levels or layers where photomasks are employed in conjunction with standard process steps to define features in the device substrate or on the layers. In the usual process, alignment marks are formed on the first level to provide means for registering the photomask for the subsequent photolithography steps. The marks usually consist of an etched pattern formed by photolithography.
Photolithographic patterns are typically made by spinning a uniform coating of photoresist over the entire wafer, exposing the photoresist with actinic light directed through a photomask, and developing the photoresist to leave a photoresist pattern.
In this description, the term photomask refers to the master mask or reticle that defines the pattern of actinic radiation incident on the photoresist. The term lithographic mask refers to the patterned photoresist which is used to define the regions where processing activity, e.g. etching, occurs in the layer masked by the photoresist. Each photomask, and the photoresist pattern produced therefrom, typically has imbedded therein a series of alignment marks, which are key ingredients in multilevel wafer fabrication. For example, if the first photoresist pattern is used to mask an oxide layer, silicon layer, or a metal layer for an etch step, the alignment marks will be transferred to the layer as an etched pattern. In this way when each sequential layer is deposited and patterned, the alignment marks from the first layer can be used to register all succeeding photomasks. Thus the desired topography can be constructed with great precision in the x-y planes. In projection lithography systems, the alignment itself is usually performed with automated tools. State of the art steppers have vision systems that easily identify the alignment marks on the wafer and automatically register the next photomask with the alignment marks previously formed.
Situations arise in semiconductor wafer fabrication where it would be advantageous for a single layer of photoresist to be exposed to more than one photomask. Among these situations are forming patterns in the third, i.e. z-, dimension, such as in forming T-gates or in forming severe undercuts. If these advantageous situations arise in the first lithographic step, it would be assumed that alignment of the second photomask to the first photomask must be accomplished without the precision alignment marks normally available in wafer fabrication. Such a process would yield significant alignment errors, and would in many instances of commercial practice be impractical.